Permitting a lawsuit against individual officials "under the circumstances of this case would impermissibly draw the court into 'the heart of executive and military planning and deliberation,'" said Collyer. She said the suit would require the court to examine national security policy and the military chain of command as well as operational combat decisions regarding the designation of targets and how best to counter threats to the United States.

"Defendants must be trusted and expected to act in accordance with the U.S. Constitution when they intentionally target a U.S. citizen abroad at the direction of the president and with the concurrence of Congress," said Collyer. "They cannot be held personally responsible in monetary damages for conducting war." The lawsuit sought unspecified damages.

Hina Shamsi, director of the ACLU National Security Project, called it a "deeply troubling decision that treats the government's allegations as proof while refusing to allow those allegations to be tested in court."

"The court's view that it cannot provide a remedy for extrajudicial killings when the government claims to be at war, even far from any battlefield, is profoundly at odds with the Constitution," said Shamsi, one of the attorneys who argued the case.